Sound reproduction device or microphone

ABSTRACT

A sound reproduction device or a microphone comprising an enclosure ( 2 ) which is made of a substantially gas permeable flexible material and an electrical transducer ( 1 ) having at least one connection ( 4, 5 ) with the enclosure, and usually two connections, to impart movement to the enclosure where the device acts as a loudspeaker or to receive movement of the enclosure to generate an electrical signal where the device acts as a microphone. The enclosure is adapted to be stressed by internal pressure and forms a concavity centered on at least one of the connections with the electrical transducer, which concavity acts as an acoustic diaphragm.

DESCRIPTION

The present invention relates to a sound reproduction device orloudspeaker and especially one employing an electro-acoustic transducer.Another aspect of the invention relates to a microphone.

Loudspeaker driver units in common use today consist of a soundgeneration device, often a coil or wire in the field of a magnet, thatoperates a rigid acoustical diaphragm such as a cone. Loudspeakers incommon use consist of one or more loudspeaker driver units mounted in arigid enclosure so that one face of the acoustical diaphragm operates onthe inside of the enclosure and other face produces the sound destinedfor the listener. The enclosure is often substantially air-impermeablealthough the enclosure may be vented to change the frequency spectrumcharacteristics of the sound produced. To date, most effort in the fieldof loudspeakers has been directed at the design of rigid enclosures andrigid acoustical diaphragms.

Whenever a diaphragm moves there is inevitably a region of compressedair at one face of the diaphragm and a region of rarefied air at theother face; indeed it is this property that is exploited in the commonloudspeaker to produce sound. However, it is necessary to prevent theregion of compressed air meeting the region of rarefied air, because ifthe two regions of air meet then they tend to cancel each other out andthe overall sound level is reduced. In the common loudspeaker theenclosure and the acoustical diaphragm are usually sealed, but sometimesvented as discussed above, in order to prevent the compressed air at oneface of the diaphragm reaching the rarefied air at the outer face of thediaphragm. In the common loudspeaker, it is usual for the enclosure andthe acoustic diaphragm to be rigid to prevent the region of compressedair expanding into the area of rarefied air by deforming the enclosureor diaphragm.

A rigid enclosure introduces problems. Rigid enclosures must be made ofmaterials such as plastic, metal, wood or wood composites, which areheavy and relatively expensive. The enclosure may be made of manyindividual pieces of material, tending to increase the cost ofmanufacture. It is sometimes necessary to reinforce the enclosures toprevent unwanted resonances, this makes the enclosure more difficult tomanufacture. When a volume of air is trapped within an enclosure ittends to act as a sort of ‘acoustical spring’ that resists the movementof the acoustical diaphragm, thereby decreasing the sensitivity of theloudspeaker and limiting its frequency response. A sound absorbingmaterial is often employed inside the enclosure to help to reduceunwanted resonances or reflections, but this can reduce the sensitivityof the loudspeaker. Sometimes the loudspeaker may be weighted in orderto further reduce unwanted resonances, but this tends to increase thecosts of manufacture and distribution and it reduces the portability ofthe loudspeaker by the end user.

A rigid acoustical diaphragm also introduces problems. The mass of thediaphragm affects the frequency response and sensitivity of theloudspeaker, low mass diaphragms tend to make loudspeakers with a widerfrequency range. Low cost diaphragm materials such as paper and canvasmust be treated to increase their rigidity and to prevent absorption ofwater from the atmosphere, this increases the mass of the diaphragm andtherefore reduces the frequency range. Rigid, low mass materials such asfiberglass or carbon fibre tend to cost more than paper or canvas andcan be harder to manufacture.

When using a rigid acoustical diaphragm and a rigid enclosure, it isnecessary for there to be a flexible coupling between the diaphragm andthe enclosure. This coupling is usually made of high performancematerial that is substantially air-impermeable and must be able towithstand bending through a small radius. The flexible coupling must beable to bend many thousands of times per hour of use and survive thelifetime of the loudspeaker.

Most rigidly encased loudspeakers do not perform well in wetenvironments, for example if they have vented enclosures then wateringress is a problem. The low cost materials used in loudspeaker conestend to absorb water, which damages them. Some loudspeaker driver unitsare provided with cones made of waterproof plastic films, but these tendto have poor sound reproduction qualities in comparison to othermaterials, and a sealed enclosure is also necessary.

Taking all these effects into account, the common loudspeaker appears tobe a less than ideal solution. It would be desirable for there to be aloudspeaker that has an acceptable frequency response and sensitivitybut avoids the restrictions of a rigid acoustical diaphragm and a rigidenclosure. That is an aim of the present invention.

Some attempts at flexible loudspeaker enclosures have been made in thepast, most notably European Patent EP 0129320A1. This patent describes aflexible enclosure for a loudspeaker, but it relies on two principles,firstly it utilises the rigid cone of a standard loudspeaker driver unitand secondly it relies on the principle that one face of the cone actson listener and the other face of the cone acts on the inside of theenclosure. The invention described in the above-mentioned patent isintended for use where it is impractical to provide a purpose-builtenclosure for a loudspeaker, such as inside voice communicationterminals.

The prior art in the form of rigidly enclosed loudspeakers are proven tobe effective in domestic and industrial environments alike but they areexpensive to manufacture and heavy, this is the opposite of what thisinvention sets out to achieve.

U.S. Pat. No. 5,108,338 proposes a balloon which has a music producingdevice, including a small speaker, affixed to the exterior wall of theinflatable balloon by way of an adhesive patch. The patch covers thespeaker and mutes the acoustic output. The balloon enclosure itself doesnot function as an acoustic diaphragm.

Accordingly, one aspect of the present invention provides a soundreproduction device comprising an enclosure and an electrical transducerconnected to the enclosure to impart movement thereto to generate soundand wherein the enclosure is made from a substantially air (gas)impermeable flexible material and wherein, for use, the enclosure isadapted to be stressed by internal pressure.

Since all sound reproducing devices (eg. loudspeakers) can be used inreverse as microphones, another aspect of the invention provides amicrophone comprising an enclosure and an electrical transducerconnected to the enclosure to generate an electrical signal in responseto movement imparted thereto from the enclosure and wherein theenclosure is made from a substantially air (gas) impermeable flexiblematerial and is adapted to be stressed by internal pressure.

More generally, the invention provides sound reproduction device or amicrophone, comprising an enclosure and an electrical transducerconnected to the enclosure to transfer movement therebetween, andwherein the enclosure is made from a substantially air (gas) impermeableflexible material and wherein the enclosure is adapted to be stressed byinternal pressure.

Part of the enclosure acts as an acoustic diaphragm. More particularlythere are usually two connections of the transducer with the enclosureand at least one of those connections coincides with the centre of aconcavity formed in the wall of the enclosure. The concavity acts as theacoustic diaphragm. The formation of the concavity and the overall shapeof the enclosure may be influenced by the configuration of the enclosurematerial. Struts, ties and gussets and/or localised reductions isflexibility of the enclosure material may be utilised to achieve thedesired enclosure configuration. The enclosure is in the form of aflexible skin which is relatively thin and conveniently in the form of afilm, say of plastics such as polyethylene or mylar. Preferably, asingle material is used for the entire enclosure. Stressing isconveniently achieved by filing, eg. inflating, the enclosure with afluid, for example a gas such as air, helium or carbon dioxide or with aliquid such as oil or water. Alternatively, a semi-solid may beemployed. The semi-solid may be an open cell foam, or a jelly-likesubstance. Access to the interior for filing/inflating is by way of asuitable closable passage which may incorporate a one-way valve.

By using a material which is inflatable, the enclosure can be collapsedand for example folded into a small space when not in use. By using amaterial which is water impermeable, the loudspeaker can be usedoutside.

Conveniently the electrical transducer connects with the enclosure atleast two locations, and preferably at two opposed locations wherebymovements are transferred at both locations. More particularly thefilling/inflation of the enclosure is such as to form two or moreconcavities so that each concavity behaves as an acoustic diaphragm.Each concavity is centred on its connection with the transducer. In thecase of a loudspeaker the movement is imparted to the enclosure by theelectrical transducers to generate sound, whereas in the case of amicrophone the movement of the enclosure is imparted to the electricaltransducer to generate an electrical signal representative of the sound.

The electro-acoustic transducer may be of moving coil, moving magnet,piezo-electric, electro-static or any other construction. A particularlysimple construction results where a piezo-electric transducer isemployed. A piezo-electric transducer has one or more rigid platesseparated by a piezo-electric material that exhibits the property ofchanging thickness when an electric field is applied to it. Accordingly,fixing opposite ends of the piezo-electric transducer to opposite partsof the enclosure provides a means of imparting the sound generatingmovement to the enclosure. The enclosure may be formed from a piece orpieces of, for example, polyethylene film or more preferably stillmetalised mylar, which are joined or formed into a closed and inflatableenvelope. Where transducers of other types are employed, means isprovided for transmitting the motion of the transducers to the flexibleenclosure at corresponding locations. Conveniently such othertransducers are removed from the enclosure and the motion transmittingmechanism routed into the enclosure with appropriate sealing so thatpressurisation of the enclosure is not jeopardised. The inflatableacoustic enclosure of the present invention may be incorporated intoother article, especially other inflatable articles.

In the following description, the invention is described with referenceto its application as a loudspeaker. For its application as a microphonethe reference to “movement being imparted to the enclosure by theelectrical transducer” should be read as “movement of the enclosurebeing imparted to the electrical transducer”. Thus, the connectionbetween the electrical transducer and the enclosure could be regardedmore generally a movement transfer mechanism.

The present invention will now be described further, by way of exampleonly, with reference to the accompanying drawings; in which:

FIG. 1 is a plan view of a self-enclosed sound reproducing device(loudspeaker) in accordance with the present invention,

FIG. 2 is a cross-sectional view on line A—A of FIG. 1,

FIGS. 3a and 3 b are a plan view and sectional side view respectively ofa self-enclosed loudspeaker of FIG. 1 and 2 before filling/inflation,

FIG. 4 is a fragmentary cross-sectional view of an embodiment of theinvention for reproducing a stereo signal,

FIG. 5 is a side view of an alternative embodiment of loudspeakeraccording to the invention showing provision of the electricaltransducer external of the housing, and

FIGS. 6a, b, c and d illustrate in further detail varies alternativemechanisms for translating mechanical movement from a extent transducerto the wall of the flexible enclosure.

Referring firstly to FIGS. 1, 2 and 3, there is illustrated a soundreproduction device according to one embodiment of the presentinvention.

FIG. 1 shows a flexible enclosure (2) composed of one or more pieces ofair-impermeable material. An electro-acoustic transducer (1) is joinedto inside the enclosure (2) at one or more locations. Any electricalwires (8) may be led out through a sealed aperture (7) in the enclosure(2). An aperture (6) is left in the enclosure (2) in order to facilitatefilling the enclosure (2) and possibly to facilitate fixing theelectro-acoustic transducer (1) to the enclosure (2), this aperture (6)is closed when the invention is in operation.

FIG. 2 shows a cross-section through the invention such as could beobtained by separating the invention along the line A—A shown in FIG. 1.FIG. 2 shows the enclosure (2) when filled with a filling (3). The joinsbetween the electro-acoustic transducer (1) and the enclosure (2) can beseen at points (4) and (5), where it can be seen that concavities areformed in the enclosure (2).

FIGS. 3a and 3 b show how an example embodiment of the invention couldbe constructed. Two circular pieces of polyeurethane film (9 above) and(10 below) of equal size are joined to each other in a continuous circle(11) around their circumferences to form the enclosure marked (2) inFIGS. 1 and 2. At the centre of the enclosure in between the two piecesof film could lie a piezo-electric transducer (1), with the bottom faceof the transducer attached to the bottom piece of film (shown as point(5) in FIG. 2) and the top face of the transducer attached to the topface of the film (shown as point (4) in FIG. 2). The wires that connectto the transducer (8) emerge from the enclosure through a sealed hole(7). When the enclosure is inflated, a sphere-like shape is formed(FIG. 1) with the exception of two substantial concavities ((4) and (5)in FIGS. 1 and 2, (9) and (10) in FIG. 3) where the film is joined tothe transducer (1). The whole arrangement could be described as a filledtoroid or a filled annulus (FIG. 1), a section through the enclosure ina plane running from the top to the bottom of the enclosure (line A—A)in FIG. 1) could he described as having the shape of a ‘figure of 8’(FIG. 2). In normal use a device of this type would be mounted,suspended or otherwise such that one of the concavities ((4) and (5) inFIG. 1) points at the listener. Prototypes closely matching thisdescription have been constructed and they perform admirably.

The invention therefore provides a low cost and easy to manufactureenclosure and acoustic diaphragm arrangement for general purpose soundreproduction. Prototypes have exhibited high sensitivity such that theoutput of a typical ‘personal stereo’ type radio, cassette or compactdisc player provides an acceptable volume for a listener placed a fewmetres from the invention, when a suitable transformer is used to matchthe output impedance of the personal stereo to that of the invention.The matching transformer would be unnecessary if a transducer of thecorrect impedance was available. The low cost of the invention wouldpermit the invention to be sold as a disposable or novelty item for usewith ‘personal stereos’ or perhaps even given away as a promotionalitem.

The invention could be used instead of rigid loudspeakers in‘portablestereo’ type radio, cassette or compact disc players where it would havethe advantage that it could be deflated while the unit is beingtransported from one location to another. An automatic or manual methodof inflating and deflating the invention could be provided for thisapplication.

Such sound reproduction devices could be used in temporary publicaddress systems in the place of stacks of heavy loudspeakers. This wouldhave the advantage of being easy to set up, take down and transportbetween locations. Current temporary public address systems can bedangerous as large stacks of heavy loudspeakers can topple and endangeranyone standing below them. The greatly reduced mass of the inventionsignificantly reduces the danger associated with toppling. If theinvention were to be filled with helium gas, the risk of toppling couldpotentially be removed altogether.

In domestic use, the invention is more versatile than rigid loudspeakersbecause it offers the option of being easily suspended from the ceilingrather than supported on the floor. The invention could also be lessobtrusive than rigid loudspeakers by using a clear film for theenclosure.

In industrial, commercial and harsh environments, the invention caneasily be suspended from the ceiling due to the low mass of theinvention. Tough materials can be used for the enclosure if there is therisk of mechanical damage; a rigid but lightweight perforated enclosurecould be provided if there is great risk of mechanical damage.

In the office environment, the ‘suspended ceiling’ scheme commonly foundin offices requires that a hole is cut in the ceiling to accept aconventional loudspeaker driver unit, the hole being the same size as orslightly smaller than the loudspeaker driver unit that is fitted. Theinvention could be used in the place of conventional loudspeaker driverunits. If there is sufficient headroom, the invention could he suspendedfrom the ceiling and it is only necessary to drill a small hole in theceiling to accommodate the electrical cable.

In wet, outdoor or humid environments, traditional loudspeakers eitherdegrade rapidly or they must be constructed in unusual ways or withunusual materials such that water ingress and water damage is minimised.The materials used in the enclosure for the invention - flexible,air-impermeable materials =- are almost always waterproof and thereforethe invention is quite suitable for operation in wet, outdoor or humidenvironments.

It is also envisaged that a stereo version of the invention could bemade by inserting two electro-mechanical transducers in the place of thesingle electro-mechanical transducer described in the embodiment of theinvention above. One transducer would be driven by signals destined forthe listener's left ear and the other transducer would be driven bysignals destined for the listener's right ear. The two transducers wouldbe bonded together along a common face and each of the other faces ofthe transducers would act on one side of the diaphragm.

FIG. 4 shows how two piezo-electric transducers might be joined togetherin an example embodiment of a stereo version of the invention. Thepiezo-electric transducers (12, 13) are joined together at (14). It maybe necessary to insert a mass in the gap (14) to improve stereoseparation. A single piezo-electric transducer with piezo-electriccoating on two sides could also perform the function of the twotransducers (12, 13)). The two transducers would then be joined to twofaces of the substantially air-impermeable membrane (4, 5) as in thefirst example embodiment of the invention. The electrical connections(15) would be routed out of the enclosure in the same way as theelectrical connections (8) are routed out of the enclosures in the firstembodiment of the design (at point 7)).

In the example embodiments of the invention it may be necessary toinsert a matching transformer in the electrical path between the devicedriving the invention and the invention. Sound amplification equipmentin common use tends to have low output impedance because loudspeakers incommon use tend to have low input impedance. Piezo-electric transducers,and therefore the example embodiments of the invention, tend to havehigh input impedances. To ensure sufficient power transfer theimpedances of the driver and the driven electrical item must be similarand a transformer is the normal method of circumvention this problem.Another method of circumventing this problem would be to sue soundamplification equipment with high output impedance. Another method ofcircumventing this problem would be to reduce the impedance of theinvention for example by using multiple piezo-electric transducers ineach embodiment and making parallel electrical connections to them, orby the use of a single piezo-electric transducer with a low impedance,or by using a single piezo-electric transducer with multiple layers ofelectrically isolated piezo-electric material and making parallelconnections to each layer, or by using a different type ofelectro-mechanical transducer with a lower characteristic impedance.

The invention has been described above with reference to the use ofpiezo-electric transducer and where the electro-acoustic transducer isdisposed within the enclosure. FIGS. 5, 6 a, 6 b, 6 c and 6 d describeembodiments where the electro-acoustic transducer is disposed externalof the enclosure and a motion transmitting mechanism is employed tocommunicate movement between the external electro-acoustic transducerand the enclosure.

FIG. 5 illustrates a general arrangement whereby the enclosure whenfilled/inflated comprises the afore-described enclosure (2) and showsthe electro-acoustic transducer (1), for example a moving coil assemblydisposed externally of the enclosure and the use of a motion transfermechanism (15) which enters the enclosure and communicates with theenclosure in two locations. In one embodiment, the electro-acoustictransducer is used to pressurise a fluid or gas which acts on theinterior of the enclosure by way of a piston/cylinder assembly. In thisway movement is imparted to the enclosure in exactly the same way aswith the piezo-electric transducer.

FIG. 6a illustrates a possible mechanical linkage for transmittingmovement to the walls of the enclosure at the centre of concavities (4)and (5) as previously described. In this instance a moving coil assembly(1′) produces relative movement between two elements (eg. an actuatorrod (17) and a hollow tube (19)). The relative motion between the twoelements is translated into movement in the diaphragm, eg. portions (4)and (5) by the illustrated mechanical linkage. A bottom pivot (21) isattached to the tube (19) and the top pivot (23) is attached to theactuator rod (17). Further pivot points (24, 25) are connected with thepivot points (21, 23) by way of limbs (26, 27) in the case of pivotpoint (24) and (28, 29) in the case of pivot point (25). The actuatorcould equally be a wire, cord or the like. Some method of sealing thesystem would be required, perhaps at the moving coil assembly. Byadjusting the dimensions of the linkages it would be possible to employa mechanical advantage. No stop will be required to prevent the coilbeing pulled out of the magnet as the mechanical linkage would preventthis. Friction must be avoided as it will generate noise which in turnwill be amplified by the diaphragm. A DC bias will be required in normaloperation in order to keep the coil in the centre of the magnet and thismay be provided electronically or it may be possible to use a spring(30) shown in dotted outline in the illustrated embodiment to achievethis, for example connected between pivot points (24) and (25)

FIG. 6b shows an alternative using a flexible linkage similar in effectto the linkage described with reference to FIG. 6a but instead a cord(17′) is employed and which runs up a tube (19′) extending from theacoustic transducer into the enclosure. A further cord or wire (31) hasits ends connected to the enclosure at the centre of concavities (4) and(5) as previously and the end of the actuator cord (17′) is connected tocord (31). Movement of cord (17′) in directions a or b is translatedinto movement in directions c or d by passing the cord (31) overrespective rollers (33, 35) carried at the end of the tube (19′). Therollers may be replaced by a smooth curved surface. This mechanism hasthe advantage of having a lower mass. A DC bias current m ay be utilisedas described above.

Referring now to FIGS. 6c and d there is illustrated a yet furtherembodiment of flexible linkage in which motion from the acoustictransducer is conveyed along cord (17″) to a roller (41) which ismounted for rotation about an axis(43) on a support taken off connectingtube (19″), thus movement of the cord (17″) in directions a or b causesmovement of the roller in either a clockwise or anti-clockwisedirection. Further cords (44, 45) connect with the roller and with thecentre of concavities (4) and (5) respectively whereby movement of thecord (17″) is translated to movement of the enclosure to generate sound.The pressurisation within the enclosure will serve as a natural returnspring centralising the mechanism.

It will be apparent from the above that numerous alternatives arepossible with regard to translating movement to the flexible enclosureand the above are merely provided by way of example only.

What is claimed is:
 1. A sound reproduction device or microphone comprising: an enclosure made from substantially gas impermeable flexible expandable material adapted to be stressed by internal pressure when filled with a filling, the enclosure having an inside surface; and an electrical transducer having at least two connections coupled to the inside surface of the enclosure, the at least two connections, with the inside surface, forming two concavities when the enclosure is expanded by internal pressure by restraining expansion of the enclosure at the at least two connections, each concavity acting as an acoustic diaphragm, and the at least two connections, with the inside surface of enclosure, serving to impart movement thereto to generate sound or to respond to movement of the enclosure to generate an electrical signal.
 2. A sound reproduction device or microphone according to claim 1, wherein the enclosure is stressed by filling with a fluid.
 3. A sound reproduction device or microphone according to claim 2, wherein the enclosure incorporates an openable and closable filling tube.
 4. A sound reproduction device according to claim 1, wherein the enclosure is stressed by the presence of a semi-solid material within the enclosure.
 5. A device according to claim 1, wherein the enclosure is collapsible into a substantially flat state corresponding to evacuation of the enclosure.
 6. A device according to claim 1, wherein the internal connections of the electrical transducer with the enclosure are by a movement transfer mechanism.
 7. A sound reproduction device according to claim 6, wherein the enclosure has two open concavities disposed in opposite relation with the movement transfer device of the electrical transducer interposed therebetween.
 8. A device according to claim 1, wherein the electrical transducer is one of a piezo-electric transducer, a moving coil, a moving magnet and a device which operates on an electro-static principle.
 9. A device according to claim 1, wherein the electrical transducer is a piezo-electric transducer and is located within the enclosure.
 10. A device according to claim 1, wherein the electrical transducer is one of a moving magnet and a moving coil and is disposed external of the enclosures and controls a movement transfer mechanism which is disposed within the enclosure and connected therewith at said two or more connections.
 11. A sound reproduction device according to claim 10, wherein the enclosure has two open concavities disposed in opposite relation with the movement transfer device of the electrical transducer interposed therebetween.
 12. A sound reproduction device according to claim 1, wherein the enclosure has two open concavities disposed in opposite relation with the electrical transducer interposed therebetween. 